Methyl conjugation is an important pathway in the metabolism of drugs, xenobiotics, endogenous compounds and neurotransmitters. The experiments proposed are designed to enhance our understanding of the biological basis for individual variations in methyl conjugation. Our laboratory has already discovered genetic polymorphisms that regulate activities of the O-methylating enzyme, catechol-O-methyltransferase (COMT) and the S-methylating enzyme, thiopurine methyltransferase (TPMT) in man. We propose to expand our studies of TPMT to include experiments with inbred strains of mice with very different TPMT activities and experiments with cultured human lymphocytes from subjects with different TPMT genotypes. In addition, our studies of methyl conjugation will be extended to include the enzyme histamine N-methyltransferase (HNMT). We have recently confurmed that HNMT activity is present in an easily accessible human tissue, the erythrocyte (RBC). We have developed a sensitive assay for the measurement of RBC HNMT activity. One goal of this proposal is to determine whether variations in the biochemical properties and regulation of HNMT in the human RBC are influenced by inheritance. It will then be determined whether the biochemical properties and regulation of HNMT in an easily accessible tissue, the RBC, are similar to and are correlated with those of HNMT in the kidney and liver, two less accessible human tissues. To accomplish these goals, HNMT activity will be measured in RBCs of first-degree ralatives, and the role of inheritance in regulating variations in the level of activity will be determined. HNMT will be purified from the human RBC, human kidney and human liver, and the biochemical properties of the enzyme in those tissues will be compared. Individual variation in RBC HNMT activities will be correlated with variations in HNMT activities in samples of human kidney and liver obtained during clinically-indicated nephrectomies and partial hepatectomies. Monoclonal and polyclonal antibodies to human HNMT will be developed to study the cellular localization of the enzyme in human organs and to study mechanisms responsible for individual variations in the level of activity of the enzyme. The results of these experiments will enhance our understanding of the biological basis for individual variations in the activities of enzymes that catalyze the S-and N-methylation of drugs, xenobiotics, and endogenous compounds.